Dehydrogenation of saturated hydrocarbons



BEST AVAILAPIE CDPY June 13, 1939. H. M. GUINOT 2,162,011

' DEHYDROGENATIION OF SATURATED HYDROCARBONS I Filed Dec. 17, 1935 A Fig. 1. I

R S amen 702 I Zi/WM 260. CHEMISTRYCARBON OMPQIUND 68g Patented June 13, 1939 UNITED STATES PATENT OFFICE DEHYDROGENATION OF SATURATED HYDROCARBONS corporation of France Application December 17, 1935, Serial No. 54,955 In France December 27, 1934 8 Claims.

It is known that by heating paraffin hydrocarbons at a high temperature in the presence of catalysts, if required, it is possible to convert them into the corresponding olefine hydrocarbons with the simultaneous evolution of hydrogen in accordance with the following typical equation:

A state of equilibrium exists for every reaction temperature: the percentage of olefine hydrocarbon formed increases with the temperature, at least between certain limits-for example between 350 and 500 C. However, it is also known that side by side with this reaction of dehydrogenation properly so called, other secondary cracking reactions may take place among the carbon atoms of the hydrocarbon molecule itself.

These secondary reactions are produced by the most diverse catalysts, for example by the metals which form the walls of the furnaces, but catalysts that are capable of directing the reaction towards the exclusive production of olefine hydrocarbons that possess the same number of carbon atoms as the saturated hydrocarbon treated are rare. Thus, in the case of the heat treatment of butane, there are generally found, in addition to butylene which is to be expected, some methane, ethane, ethylene, propane, propylene and even carbon.

For the purpose of effecting by catalysis the simple dehydrogenation of the paraflln hydrocarbons, substances such as the air-dried gel of chromic acid having the formula Cr(OH): has been proposed, which is capable of giving interesting results. Unfortunately its activity is of short duration and on working at only moderately high temperature of the order of 450-500 C., that is to say, at the only temperatures which enable a substantial coeflicient of conversion to be attained, this activity is rapidly extinguished, i. e., in less than four hours (Ind. Eng. Chem. 1933, page 54).

The present invention has among its objects to provide improvements which will enable the dismium oxide of the formula CrO, and by trivalent chromium oxide is meant chromium oxide of the formula CrzOa.

Catalysts with a base of oxide of divalent chromium can be used up to the temperature of 500 to 525 C. They direct the reaction solely towards the formation of the olefine hydrocarbons that correspond to the saturated hydrocarbons treated. Their activity keeps constant for more than a hundred hours and it then decreases slowly, but the original activity can be recovered by heating the catalyst for a few hours at 550 C. in the presence of hydrogen.

Catalysts consisting of a mixture of divalent and trivalent chromium oxides have similar properties.

According to the invention, furthermore, the unconverted saturated hydrocarbon is completely recovered and is continuously returned into the circuit after partial or complete separation of the hydrogen with which it is mixed.

These two features of the invention may be applied separately or in combination. In combination they are particularly advantageous, because practically the entire conversion of a given saturated hydrocarbon into the olefine with the same number of carbon atoms, or into derivatives of such olefine, can be carried out.

The following examples, which are in no way limitative, will enable the manner in which the invention can be carried out to be well understood and will now be described with reference to the accompanying diagrammatic drawing.

Example I In a furnace A of fused silica (Figure 1) having a capacity of 10 litres, are placed lozenges of a catalyst consisting of 50 per cent. of oxide of divalent chromium and 50 per cent. of infusorial earth. The temperature of the furnace being kept at 500 C., propane is passed into it at a velocity of 1000 litres per hour through the pipe G. The gases leaving the furnace through the pipe M contain 11 per cent. of propylene. They are passed through a pipe M into an apparatus B where they are treated with a solution of hypochlorous acid which absorbs the propylene with the formation of chlorohydrin of propylene glycol.

The residual gas then consists of a mixture of unconverted propane and hydrogen, a mixture which it is not advisable to send back directly in its entirety into the dehydrogenation furnace, because it is there a question of an equilibrium reaction and the presence of a large proportion of hydrogen is undesirable.

It is therefore first of all necessary to effect the separation of the propane from the hydrogen. This is done by dissolving the propane in a solvent which has a moderate boiling point and which can, for example, be introduced through the pipe 0 to the top of a washing tower C into the bottom of which the mixture of propane and hydrogen is passed, through the pipe N. The hydrogen, which is insoluble in the solvent, is removed through the pipe D; the solvent, which is charged with propane, is passed through the pipe P into an apparatus Q where the propane is expelled from the solvent by heating slightly at E and is sent back to the dehydrogenation furnace through the pipe F; the recovered solvent is led back to the tower C by the pipe R, the pump S (where fresh solvent may be added from time to time to replace losses) and the pipe 0.

Example II In the catalytic furnace A (Figure 2.) having a capacity of [0 litres, are placed lozenges of a catalyst consisting of the same mixture as in Example I. The furnace being heated to 475 C., butane is passed into it at a velocity of 800 litres per hour. The gases leaving the furnace contain 16 per cent. of butylene; there is no formation either of propane or of propylene and the quantity of ethylene found in the gas is less than 0.18 per cent.

It is to be noted that the proportion of butylene thus found corresponds very nearly to the theoretical equilibrium for this temperature.

The gases leaving the furnace are passed into the apparatus B where the butylene is absorbed by some reagent adapted to hydrate the double bond and to form the corresponding secondary butyl alcohol. This reagent will be, for example dilute sulphuric acid.

Instead of treating the whole of the gases leaving the apparatus B as in Example I, it is possible to treat only a part thereof; if the proportion of hydrogen in the mixture is not too high, the saturated hydrocarbon can be dehydrogenated by giving it a reasonable coeflicient of conversion at each passage through the furnace. Thus, in the case of the butane considered in the present example, 16 per cent. conversion into butylene is obtained at 475 C. with the pure saturated hydrocarbon, whilst a gas containing 17.6 per cent. of hydrogen gives 10.5 per cent. conversion into butylene at the same temperature and a gas with 50 per cent. of hydrogen gives 4.5 per cent. conversion into butylene at each passage. To take advantage of such conditions, the apparatus can. be supplemented as shown in Figure 2 and the operation then is as follows: at the exit from the apparatus B, only a part of the residual gas is taken and is treated with a suitable solvent for example Vaseline oil-in the washing tower C into which it has been passed through the pipe N; thehydrogen issues through the pipe D and the butane recovered is returned to the dehydrogenation furnace through the pipe F, after being freed from the solvent by slight heating in the apparatus Q; at the same time, a suitable quantity of fresh butane is passed into the circuit through the pipe G.

The other part of the gas leaving the apparatus B is returned directly into the furnace through the pipe K together with the hydrogen which it contains. In working in accordance with this modification, the gas that has been treated in the washing tower C is richer in hydrogen; this simplifies the work of recovering the butane; on the other hand, with the same catalytic furnace, the production of butylene per passage is lower.

Owing to this recovery of the unconverted butane and the fact that the catalyst chosen does not effect the destruction of the molecule of the saturated hydrocarbon treated, it is possible to carry out practically the entire conversion of the saturated hydrocarbon into an alcohol having the same number of carbon atoms; this result has never been obtained previously.

Example III In a furnace that is similar to the one used in the preceding examples, there are placed 10 litres of a catalyst consisting of lozenges formed of 50 per cent. of oxide of divalent chromium and 50 per cent. of oxide of trivalent chromium without a carrier. The temperature of the furnace being kept at 500 0., isohexane (dimethylpropylmethane), having a boiling point of 60 C. at the ordinary pressure and a density of 0.65, is passed into it at the rate of 3000 gms. per hour. A mixture is collected at the exit from the furnace containing 30 per cent hexenes; the boiling point of the condensed product is from 57 to 61 C. at atmospheric pressure; there has been no formation of the C4 or C5 hydrocarbons and the total quantity of ethylene, propylene and propane formed is less than 0.7 per cent. the weight of the isohexane supplied.

In view of the fact that thesusceptibility of hydrocarbons to cracking increases in accordance with their molecular weight, the essentially dehydrogenating reaction thus obtained with isohexane is quite remarkable.

Here again it is advantageous to recover the unconverted isohexane for the purpose of returning it into the circuit. As isohexane is a hydrocarbon which is liquid at ordinary temperature, the separation from the hydrogen is preferably effected by liquefaction.

The preceding examples are not limitative. Thus the operation can be carried out at a pressure other than atmospheric, for example by fitting a fan V (Figure 2) that is adapted to produce a slight vacuum in the dehydrogenation furnace A and an excess of pressure in the rest of the circuit; this excess of pressure gives the advantage of facilitating the hydration of the olefine that is formed and the subsequent separation of the residual saturated hydrocarbon from the hydrogen.

It is likewise to be understood that the olefine obtained may be used for manufactures other than those of alcohols and of the chlorohydrins mentioned in the examples.

What I claim is:

1. The cyclic dehydrogenation of saturated hydrocarbons consisting in subjecting the hydrocarbon to catalytic dehydrogenation with a catalyst consisting essentially of the oxide of divalent chromium, separating the dehydrogenation product from the"urichanged saturated hydrocarbon and the hydrogen of reaction, removing the hydrogen from the unchanged saturated hydrocarbon by means of a solvent therefor, liberating the absorbed hydrocarbon from the solvent and returning it to the dehydrogenation stage.

2. The cyclic dehydrogenation of saturated hydrocarbons consisting in subjecting the hydrocarbon to catalytic dehydrogenation with a catalyst consisting essentially of the oxide of divalent chromium, separating the dehydrogenation prodnot from the unchanged saturated hydrocarbon and the hydrogen oi reaction, removing the hydrogen from the unchanged saturated hydrocarbon by liquifaction oi the latter, vaporising the hydrocarbon and returning it to the dehydrogenation stage.

3. The cyclic dehydrogenation of saturated hydrocarbons consisting in subjecting the hydrocarbon to catalytic dehydrogenation with a catalyst consisting essentially of the oxide of divalent chromium, separating the dehydrogenation product from the unchanged saturated hydrocarbon and the hydrogen of reaction, returning a part of the mixture of unchanged saturated hydrocarbon and the hydrogen of reaction direct to the dehydrogenation stage, removing from the remainder of the mixture the hydrogen content and returning the hydrocarbon content to the dehydrogenation stage.

4. A process for the conversion of saturated paraflin hydrocarbons into thecorresponding olefine hydrocarbons comprising passing the saturated hydrocarbons over a dehydrogenation catalyst consisting essentially oi ordde of divalent chromium.

5. A process for the'conversion of saturated paraflin hydrocarbons into the corresponding olefine hydrocarbons comprising passing the saturated hydrocarbons over a dehydrogenation catalyst consisting essentially of oxide oi divalent chromium at a temperature of approximately 475-525 C.

6. The cyclic dehydrogenation of saturated paramn hydrocarbons comprising subjecting the hydrocarbons to catalytic dehydrogenation with a catalyst consisting essentially of an oxide of divalent chromium, separating the dehydrogenation product from the unchanged saturated hydrocarbon and the hydrogen of reaction, removing hydrogen from the unchanged saturated hydrocarbon, and returning the latter to the dehydrogenation stage. i

7. The catalytic conversion 01 saturated paraffin hydrocarbons into unsaturated hydrocarbons having the same number of carbon atoms, with substantially complete absence of secondary reaction products, comprising subjecting the saturated hydrocarbons to the action of heat and a catalyst comprising essentially an oxide of divalent chromium.

8. A process for the conversion of saturated paramn hydrocarbons into the corresponding olefine hydrocarbons comprising passing the saturated hydrocarbons over a dehydrogenation catalyst consisting essentially of a mixture of the oxides of divalent and trivalent chromium.

. HENRI MARTIN GUINOT. 

